The present invention relates to the development of reconfigurable artificial magnetic conductor (RAMC) surfaces for low profile antennas. This device operates as a high-impedance surface over a tunable frequency range, and is electrically thin relative to the wavelength of interest, xcex.
A high impedance surface is a lossless, reactive surface, realized as a printed circuit board, whose equivalent surface impedance is an open circuit which inhibits the flow of equivalent tangential electric surface currents, thereby approximating a zero tangential magnetic field. A high-impedance surface is important because it offers a boundary condition which permits wire antennas (electric currents) to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface ( less than xcex/100 away). The opposite is true if the same wire antenna is placed very close to a metal or perfect electric conductor (PEC) surface. It will not radiate efficiently. The radiation pattern from the antenna on a high-impedance surface is confined to the upper half space above the high impedance surface. The performance is unaffected even if the high-impedance surface is placed on top of another metal surface. The promise of an electrically thin, efficient antenna is very appealing for countless wireless device and skin-embedded antenna applications.
One embodiment of a thin, high-impedance surface 100 is shown in FIG. 1. It is a printed circuit structure forming an electrically thin, planar, periodic structure, having vertical and horizontal conductors, which can be fabricated using low cost printed circuit technologies. The high-impedance surface 100 includes a lower permittivity spacer layer 104 and a capacitive frequency selective surface (FSS) 102 formed on a metal backplane 106. Metal vias 108 extend through the spacer layer 104, and connect the metal backplane to the metal patches of the FSS layer. The thickness of the high impedance surface 100 is much less than xcex/4 at resonance, and typically on the order of xcex/50, as is indicated in FIG. 1.
The FSS 102 of the prior art high impedance surface 100 is a periodic array of metal patches 110 which are edge coupled to form an effective sheet capacitance. This is referred to as a capacitive frequency selective surface (FSS). Each metal patch 110 defines a unit cell which extends through the thickness of the high impedance surface 100. Each patch 110 is connected to the metal backplane 106, which forms a ground plane, by means of a metal via 108, which can be plated through holes. The spacer layer 104 through which the vias 108 pass is a relatively low permittivity dielectric typical of many printed circuit board substrates. The spacer layer 104 is the region occupied by the vias 108 and the low permittivity dielectric. The spacer layer is typically 10 to 100 times thicker than the FSS layer 102. Also, the dimensions of a unit cell in the prior art high-impedance surface are much smaller than xcex at the fundamental resonance. The period is typically between xcex/40 and xcex/12.
Another embodiment of a thin, high-impedance surface is disclosed in U.S. patent application Ser. No. 09/678,128, entitled xe2x80x9cMulti-Resonant, High-Impedance Electromagnetic Surfaces,xe2x80x9d filed on Oct. 4, 2000, commonly assigned with the present application and incorporated herein by reference in its entirety. In that embodiment, an artificial magnetic conductor is resonant at multiple resonance frequencies. That embodiment has properties of an artificial magnetic conductor over a limited frequency band or bands, whereby, near its resonant frequency, the reflection amplitude is near unity and the reflection phase at the surface lies between +/xe2x88x9290 degrees. That embodiment also offers suppression of transverse electric (TE) and transverse magnetic (TM) mode surface waves over a band of frequencies near where it operates as a high impedance surface.
Another implementation of a high-impedance surface, or an artificial magnetic conductor (AMC), which has nearly an octave of +/xe2x88x9290xc2x0 reflection phase, was developed under DARPA Contract Number F19628-99-C-0080. The size of this exemplary AMC is 10 in. by 16 in by 1.26 in thick (25.4 cmxc3x9740.64 cmxc3x973.20 cm). The weight of the AMC is 3 lbs., 2oz. The 1.20 inch (3.05 cm) thick, low permittivity spacer layer is realized using foam. The FSS has a period of 298 mils (0.757 cm), and a sheet capacitance of 0.53 pF/sq.
The measured reflection coefficient phase of this broadband AMC, referenced to the top surface of the structure is shown in FIG. 2 as a function of frequency. A xc2x190xc2x0 phase bandwidth of 900 MHz to 1550 MHz is observed. Three curves are traced on the graph, each representing a different density of vias within the spacer layer. For curve AMC1-2, one out of every two possible vias is installed. For curve AMC1-4, one out of every four vias is installed. For curve AMC1-18, one out of every 18 vias is installed. As expected from the effective media model described in application Ser. No. 09/678,128, the density of vias does not have a strong effect on the reflection coefficient phase.
Transmission test set-ups are used to experimentally verify the existence of a surface wave bandgap for this broadband AMC. In each case, the transmission response (S21) is measured between two Vivaldi-notch radiators that are mounted so as to excite the dominant electric field polarization for transverse electric (TE) and transverse magnetic (TM) modes on the AMC surface. For the TE set-up, the antennas are oriented horizontally. For the TM set-up, the antennas are oriented vertically. Absorber is placed around the surface-under-test to minimize the space wave coupling between the antennas. The optimal configurationxe2x80x94defined empirically as xe2x80x9cthat which gives the smoothest, least-noisy response and cleanest surface wave cutoffxe2x80x9dxe2x80x94is obtained by trial and error. The optimal configuration is obtained by varying the location of the antennas, the placement of the absorber, the height of absorber above the surface-under-test, the thickness of absorber, and by placing a conducting foil xe2x80x9cwallxe2x80x9d between layers of absorber. The measured S21 for both configurations is shown in FIG. 3. As can be seen, a sharp TM mode cutoff occurs near 950 MHz, and a gradual TE mode onset occurs near 1550 MHz. The difference between these two cutoff frequencies is referred to as a surface wave bandgap. This measured bandgap is correlated closely to the +/xe2x88x9290-degree reflection phase bandwidth of the AMC.
The resonant frequency of the prior art AMC, shown in FIG. 1, is given by Sievenpiper et. al. (IEEE Trans. Microwave Theory and Techniques, Vol. 47, No. 11, November 1999, pp. 2059-2074), (Also see Dan Sievenpiper""s dissertation, High Impedance Electromagnetic Surfaces, UCLA, 1999) as ƒo=1/(2xcfx80{square root over (LC)}) where C is the equivalent sheet capacitance of the FSS layer in Farads per square, and L=xcexcoh is the permeance of the spacer layer, with h denoting the height or thickness of this layer.
In most wireless communications applications, it is desirable to make the antenna ground plane as small and light weight as possible so that it may be readily integrated into physically small, light weight platforms such as radiotelephones, personal digital assistants and other mobile or portable wireless devices. The relationship between the instantaneous bandwidth of an AMC with a non-magnetic spacer layer and its thickness is given by       BW          f      0        =      2    ⁢          xe2x80x83        ⁢    π    ⁢          h              λ        0            
where xcex0 is the free space wavelength at resonance where a zero degree reflection phase is observed. Thus, to support a wide instantaneous bandwidth, the AMC thickness must be relatively large. For example, to accommodate an octave frequency range (BW/ƒ0=0.667), the AMC thickness must be at least 0.106xcex0, corresponding to a physical thickness of 1.4 inches at a center frequency of 900 MHz. This thickness is too large for many practical applications.
Accordingly, there is a need for an artificial magnetic conductor, which allows for a wider frequency coverage for a given AMC thickness.
The present invention provides a means to electronically adjust or tune the resonant frequency, ƒo, of an artificial magnetic conductor (AMC) by controlling the effective sheet capacitance C of its FSS layer.
By way of introduction only, one present embodiment provides an artificial magnetic conductor (AMC) which includes a frequency selective surface (FSS) including a single layer of conductive patches, with one group of conductive patches electrically coupled to a reference potential and a second group of conductive patches forming bias nodes. The FSS further includes voltage variable capacitive elements coupling patches of the one group of conductive patches with patches of the second group and decoupling resistors between the patches of the second group.
Another embodiment provides an AMC which includes a ground plane, a spacer layer disposed adjacent the ground plane and a plurality of vias in electrical contact with the ground plane and extending from a surface of the ground plane in direction of the spacer layer. The AMC further includes a FSS disposed on the spacer layer and including a periodic pattern of bias node patches alternating with ground node patches. The ground node patches are in electrical contact with respective vias of the plurality of vias. The AMC further includes components between selected bias node patches and ground node patches, the components having a capacitance which is variable in response to a bias voltage. The AMC still further includes a network of bias resistors between adjacent bias node patches.
Another embodiment provides an AMC which includes a means for forming a backplane for the AMC and a FSS including means for varying capacitance of the FSS. The AMC further includes a spacer layer separating the means for forming a back plane and the FSS. The spacer layer includes a plurality of vias extending substantially normal to the FSS.
Another embodiment provides an AMC including a FSS including a ferroelectric thin film, a first layer of conductive patches on one side of the ferroelectric thin film, and a second layer of conductive patches on a second side of the ferroelectric film. The patches of the second layer overlapping at least in part patches of the first layer. The AMC further includes a spacer layer including first vias associated with patches of the first layer and second vias associated with patches of the second layer and a backplane conveying bias signals to the first vias and the second vias.
Still another embodiment provides an artificial magnetic conductor (AMC) which includes a frequency selective surface (FSS) having a pattern of conductive patches, a conductive backplane structure, and a spacer layer separating the FSS and the conductive backplane structure. The spacer layer includes conductive vias associated with some but not all patches of the pattern of conductive patches to create a partial forest of vias in the spacer layer.
The foregoing summary has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.